Drilling fluids

ABSTRACT

The invention is a novel drilling fluid additive comprising particles of elastomer which are capable of swelling upon contact with a hydrocarbon fluid. The swelling of the elastomer is gradual, allowing the elastomer to reach the lost circulation zone before significant swelling occurs. Once in the lost circulation zone, the polymer expands sealing off the lost circulation zone. A method for preventing drilling fluid loss is also provided.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/177,640, filed Jan. 24, 2000.

The invention relates to improved drilling fluids which contain fineparticles of crosslinked elastomer. The elastomer acts as a pluggingagent thereby preventing loss of the drilling fluid to a porousformation. A method for preventing loss of drilling fluids is alsoprovided.

Drilling fluids, or drilling muds as they are sometimes called, areslurries used in the drilling of wells in the earth for the purpose ofrecovering hydrocarbons and other fluid materials. Drilling fluids havea number of functions, the most important of which are: lubricating thedrilling tool and drill pipe which carries the tool, removing formationcuttings from the well, counterbalancing formation pressures to preventthe inflow of gas, oil or water from permeable rocks which may beencountered at various levels as drilling continues, and holding thecuttings in suspension in the event of a shutdown in the drilling andpumping of the drilling fluid.

For a drilling fluid to perform these functions and allow drilling tocontinue, the drilling fluid must stay in the borehole. Frequently,undesirable formation conditions are encountered in which substantialamounts or, in some cases, practically all of the drilling fluid may belost to the formation. Drilling fluid can leave the borehole throughlarge or small fissures or fractures in the formation or through ahighly porous rock matrix surrounding the borehole.

Most wells are drilled with the intent of forming a filter cake ofvarying thickness on the sides of the borehole. The primary purpose ofthe filter cake is to reduce the large losses of drilling fluid to thesurrounding formation. Unfortunately, formation conditions arefrequently encountered which may result in unacceptable losses ofdrilling fluid to the surrounding formation despite the type of drillingfluid employed and filter cake created.

A variety of different substances are now pumped down well bores inattempts to reduce the large losses of drilling fluid to fractures andthe like in the surrounding formation. Different forms of cellulose arethe preferred materials employed. Some substances which have been pumpedinto well bores to control lost circulation are: almond hulls, walnuthulls, bagasse, dried tumbleweed, paper, coarse and fine mica, and evenpieces of rubber tires. These and other prior art additives aredescribed in U.S. Pat. No. 4,498,995.

Another process that is employed to close off large lost circulationproblems is referred to in the art as gunk squeeze. In the gunk squeezeprocess, a quantity of a powdered bentonite is mixed in diesel oil andpumped down the well bore. Water injection follows the bentonite anddiesel oil. If mixed well, the water and bentonite will harden to form agunky semi-solid mess, which will reduce lost circulation. Problemsfrequently occur in trying to adequately mix the bentonite and water inthe well. The bentonite must also be kept dry until it reaches thedesired point in the well. This method is disclosed in U.S. Pat. No.3,082,823.

Many of the methods devised to control lost circulation involve the useof a water expandable clay such as bentonite which may be mixed withanother ingredient to form a viscous paste or cement. U.S. Pat. No.2,890,169 discloses a lost circulation fluid made by forming a slurry ofbentonite and cement in oil. The slurry is mixed with a surfactant andwater to form a composition comprising a water-in-oil emulsion havingbentonite and cement dispersed in the continuous oil phase. As thiscomposition is pumped down the well bore, the oil expands andflocculates the bentonite which, under the right conditions, forms afilter cake on the well bore surface in the lost circulation area.Hopefully, the filter cake will break the emulsion causing theemulsified water to react with the cement to form a solid coating on thefilter cake. But such a complex process can easily go wrong.

U.S. Pat. No. 3,448,800 discloses another lost circulation methodwherein a water soluble polymer is slurried in a nonaqueous medium andinjected into a well. An aqueous slurry of a mineral material such asbarite, cement or plaster of paris is subsequently injected into thewell to mix with the first slurry to form a cement-like plug in the wellbore.

U.S. Pat. No. 4,261,422 describes the use of an expandable clay such asbentonite or montmorillonite which is dispersed in a liquid hydrocarbonfor injection into the well. After injection, the bentonite ormontmorillonite will expand upon contact with water in the formation.Thus, it is hoped that the expanding clay will close off water producingintervals but not harm oil producing intervals.

A similar method is disclosed in U.S. Pat. No. 3,078,920 which uses asolution of polymerized methacrylate dissolved in a nonaqueous solventsuch as acetic acid, acetic anhydride, propionic acid and liquidaliphatic ketones such as acetone and methyl-ethyl ketone. Themethacrylate will expand upon contact with formation water in thewater-producing intervals of the well.

It has also been proposed to mix bentonite with water in the presence ofa water-soluble polymer which will flocculate and congeal the clay toform a much stronger and stiffer cement-like plug than will form ifbentonite is mixed with water. U.S. Pat. No. 3,909,421 discloses such afluid made by blending a dry powdered polyacrylamide with bentonitefollowed by mixing the powder blend with water. U.S. Pat. No. 4,128,528claims a powdered bentonite/polyacrylamide thickening compositionprepared by mixing a water-in-oil emulsion with bentonite to form apowdered composition which rapidly becomes a viscous stiff material whenmixed with water. U.S. Pat. Nos. 4,503,170; 4,475,594; 4,445,576;4,442,241 and 4,391,925 teach the use of a water expandable claydispersed in the oily phase of a water-in-oil emulsion containing asurfactant to stabilize the emulsion and a polymer dispersed in theaqueous phase. When the emulsion is sheared, it breaks and a bentonitepaste is formed which hardens into a cement-like plug. The patentdiscloses the use of such polymers as polyacrylamide, polyethylene oxideand copolymers of acrylamide and acrylic or methacrylic acid.

A group of oil absorbent polymers is disclosed in U.S. Pat. Nos.4,191,813; 4,263,407; 4,384,095 and 4,427,793. U.S. Pat. No. 4,191,813discloses lightly crosslinked copolymers containing at least 40% byweight of vinylbenzyl chloride, the balance of monomers, if any,comprising a major portion of aromatic monomers, with the copolymerbeing crosslinked in a swollen state by a Lewis acid catalyst. Thepreferred comonomers are one or more of styrene, divinylbenzene andacrylonitrile. U.S. Pat. No. 4,263,407 discloses similar copolymerswherein the copolymer is post-crosslinked in a swollen state in thepresence of a Friedel-Crafts catalyst with a crosslinker selected from apolyfunctional alkylating or acylating agent and a sulfur halide.

Another group of highly hydrocarbon absorbent copolymers is disclosed inU.S. Pat. Nos. 4,384,095 and 4,427,793. They describe a crosslinkedlinear addition copolymer which contains repeating units of vinylbenzylalcohol and at least one other alpha, beta-monoethylenically unsaturatedmonomer different from vinylbenzyl alcohol, wherein the vinylbenzylalcohol units comprise about 0.5% to about 20% by weight of the linearpolymer. The preferred comonomers are styrene, methylmethacrylate,vinyltoluene and vinylpyridine. The copolymers disclosed in all four ofthese patents absorb from two to ten times their weight in hydrocarbonsand may swell up to ten times their original volume.

Oleophilic polymers for separating oil from water which show significantswelling in volume upon absorption of oil are described in U.S. Pat. No.4,172,031. These polymers include polymers of styrenes and substitutedstyrenes, polyvinyl chloride copolymers of vinylchloride such as acopolymer of 60 wt % vinylchloride and 40 wt % vinylacetate, polymersand copolymers of vinylidene chloride and acrylonitrile, and acrylicpolymers such as polymers of methylmethacrylate and ethylacrylate,styrene and divinylbenzene copolymers and alkyl styrene polymers andcopolymers. The reference discloses that these polymers show significantswelling in volume upon absorption of oil.

U.S. Pat. No. 4,633,950 discloses the use of oil swelling polymers toreduce lost circulation in drilling fluids. In this patent, the polymersare introduced in an aqueous solution to prevent absorption of thehydrocarbon fluid until the polymers reach the well head.

While the above inventions purport to be effective in reducing loss ofdrilling fluids, there continues to be a need for effective andinexpensive additives for well bore fluids which can prevent or stop theloss of the fluids into the subterranean formation.

SUMMARY OF THE INVENTION

The invention relates to an improved additive for a drilling fluid whichsignificantly reduces the loss of fluid to the surrounding subterraneanstructure while maintaining the lubricity of the drilling fluid. Thenovel additive comprises finely ground elastomer particles.

Loss of drilling fluid occurs when drilling fluid seeps into thesubterranean formation through holes, fractures or fissures in theformation. The region in the well where this occurs is referred to asthe lost circulation zone. When elastomer particles are added to adrilling fluid, they form a seal in the lost circulation zone byexpanding in size to seal the fractures or fissures. This preventsfurther loss of drilling fluid into the formation.

In contrast to other elastomer based additives discussed above, thepresent system uses fine elastomer particles which are easily pumpedinto the well but are capable of swelling to 80% to 140% more than theiroriginal size when exposed to hydrocarbon fluids. Their ability to swellmakes the elastomer particles extremely effective at preventing loss ofdrilling fluid into the subterranean formation. When the elastomerparticles are used with hydrocarbon-based drilling fluids or with ahydrocarbon additive, particles gradually expand, allowing the materialto be easily pumped before significant expansion of the particlesoccurs. That most of the swelling occurs after the elastomer particlesare in the desired location in the well eliminates the need forprotective coatings or an aqueous pill to prevent swelling of thepolymer until it has reached the desired region in the well.

Another advantage of the elastomer particles is their very low densityas compared to conventional materials used to prevent fluid loss. Thelow density of the elastomer particles allows them to be used in higheramounts than conventional fluid loss materials.

The fluid loss prevention system of the invention require no additionaladditives other than those normally encountered in drilling fluid suchas diesel, oil, other hydrocarbon fluid and water. The elastomerparticles are easily handled and present little if any industrialhazards. Moreover, since the preferred source of the fluid loss agentsof the invention are prepared from recycled tires, the inventionpresents a way to effective use the old tire materials rather thanallowing them to collect to form hazardous waste sites.

The invention also relates to a method for reducing drilling fluid lossby adding finely ground elastomer particles to a drilling fluid in anamount sufficient to block the flow of fluid into the subterraneanformation.

DETAILED DESCRIPTION OF THE INVENTION

The drilling fluid additive of the present invention comprise particlesof elastomers which exhibit swelling when contacted with hydrocarbonfluids, yet do not dissolve in hydrocarbon fluids. This can be achievedby using crosslinked elastomers which swell but do not dissolve in thepresence of a hydrocarbon fluid. The swelling elastomers are such thatlittle, if any, swelling occurs until the polymer has reached the lostcirculation zone in the well to be sealed. This is accomplished withoutresort to coatings on the polymer or suspending the polymer with aqueouspill.

The preferred elastomer used in the practice of this invention is crumbrubber. Crumb rubber generally comprises rubber derived from recycledtires or scrap material from the manufacture of tires. Crumb rubberparticles arc generally of ⅜-inch or less in size. In the practice ofthe invention, the crumb rubber should have a mixture of particle sizesranging of from 1 to 400 microns, preferably 20 to 400 microns. In thepreferred embodiment, the material should include particles of varyingdiameters within the range stated above.

Crumb rubber can be prepared several different ways. In the firstmethod, whole tires are cryogenically frozen and then shattered in ahammer mill to break down the tire into the desired particle sizes andto remove the steel and fibers from the tire. A second method involvesphysically tearing the tire apart and removing the unwanted steel andfibers. In this process, the tire is continuously milled until thedesired particle sizes are obtained. Another source of crumb rubber isthe bushings that remain as the tire is manufactured or remanufactured.

One key feature of crumb rubber that makes it useful in the practice ofthe invention is its ability to expand up to 140% of its original sizewhen exposed to hydrocarbon fluids and temperatures. As shown in theexamples below, when the elastomer particles are exposed to hydrocarbonfluids and temperatures typically encountered in a well (200° F. to 300°F.) the particles expand to over 140% of their original size. The amountof expansion is dependent upon the hydrocarbon used and the temperaturein the well.

The expansion of the elastomer is not immediate, often taking severalhours before a significant increase is seen. The delay in expansion ofthe crumb rubber means that the crumb rubber can be easily pumped down awell bore without resort to coatings or the use of an aqueous pill. Thecrumb rubber can then flow into the pores and cracks. Once in the cracksand pores, the crumb rubber will expand to fill the cracks and poreswithout dissolving into the drilling fluid. Thus, while crumb rubber isthe preferred elastomer in the practice of the invention, any elastomerwhich exhibits the same swelling and solubility characteristics as crumbrubber may be used.

Another feature of the present invention is the relatively low densityof the elastomer particles, relative to the fluid. This allows a higheradditive loading without adversely affecting the properties of thedrilling fluid. For example, the elastomers of the invention can be usedin amounts up to 100 lbs. per barrel (ppb)whereas conventional fluidloss additives can be used in amounts ranging from 5 to 20 lbs. perbarrel. The ability to use higher additive loadings means that more ofthe elastomer particles are present in the fluid to fill and plug theopenings into the subterranean formation.

This last feature is particularly important in deepwater operationswhere the drilling fluids used require a narrow density range. Typicallythe fluids used in these applications have a density of from 9.5 to 10.5pounds per gallon (ppg). The elastomer particles of the invention have adensity of from 8.5 to 10.5 ppg. Thus, the addition of the additives ofthe invention do not adversely affect the density drilling muds.

Illustrative hydrocarbon fluids useful in this invention include crudeoil, diesel oil, kerosene, mineral oil, gasoline, naptha, toluene,ethylenedichloride and mixtures thereof. In addition, synthetic oilssuch as those derived from olefins, linear α-olefins, poly α-olefins,internal esters and ethers may be used. Because of economics,availability at any drilling site and performance, diesel oil is mostpreferred. Synthetic oils, however, are preferred where environmentalimpact is a concern.

The drilling fluid additives of the present invention can be used inboth hydrocarbon based and aqueous or water based drilling fluids. Ifpolymer expansion is needed in an aqueous system, a hydrocarbon fluidmust be added to the elastomer particles to achieve the desiredexpansion. It has been observed, however, that improved fluid loss canbe achieved in aqueous drilling fluids without adding a hydrocarbonfluid. The improved fluid loss reduction is achieved by the ability touse higher amounts of particles

One method for practicing this invention involves the injection of adiscrete pill of drilling fluid containing the drilling fluid additivesof the invention in a sufficient amount to seal off the lost circulationzone. This pill is then forced down to the lost circulation zone. Theelastomer particles then fill the holes and fractures preventing loss ofthe fluid. Depending upon the polymer and the composition of thedrilling fluid, about two to about 250 pounds of polymer per barrel offluid can be placed in the pill. Methods for introducing the pillcontaining the drilling fluid additive of the invention are well knownto those in the art.

Other matter may be added to the pill to enhance the sealing propertiesof the fluid. For example, cellulose fiber from plant matter such aspeanut shells, sugar cane husks or bagasse, almond shells, walnutshells, dried tumbleweed and paper, may be added to the pill. Bridgingmaterials such as calcium carbonate may also be added. Coarse and finemica can also be used.

To help maintain the seal established by the polymer containing pill andto prevent loss to new fractures, the polymer of the invention can becontinuously added to the drilling fluid. In these cases, the polymershould be added at a rate of 100 to 250 pounds per hour to the drillingfluid.

EXAMPLES Example 1

In this example, samples of crumb rubber were exposed to varioushydrocarbon fluids to measure the degree of expansion over time. In eachexperiment, 20 mls of the base fluid were added to a 150 mm test tube.To this was added 2.29 gms of crumb rubber. The tube was then shaken toset the crumb rubber. The total height of the fluid and crumb rubber wasmeasured at 108 mm. The height of the rubber in each sample was 33 mm.The hydrocarbon fluids used in these tests were two commercial internalolefin fluids, a linear α-olefin fluid and #2 diesel.

The test tube was then placed in a Baroid 500 ml static-aging cell whichwas then pressurized to 300 psi with nitrogen. The cell was then placedin an oven at the temperatures noted in the tables and static-aged forthree days. A duplicate sample was static-aged for seven days.

After static-aging, the test tubes were removed from the test cells andthe height of the rubber was measured. The amount of expansion wascalculated using the formula (“height after aging/33)−1.” The resultsare reported in Tables 1 and 2.

TABLE 1 Solubility and Percent Expansion of NER PipeRubber after threeday tests Temperature IO Base #1 LAO Base IO Base #2 #2 Diesel 200° F.103%  97% 127%  158% 250° F. 109% 109% 97% 152% 300° F.  112%*  112%*97%  158%* *Denotes partial solubility of PipeRubber in Base Fluid.

TABLE 2 Solubility and Percent Expansion of NER PipeRubber after sevenday tests Temperature IO Base #1 LAO Base IO Base #2 #2 Diesel 200° F. 82% 97% 103% 106% 250° F. 112% 118%  118% 158% 300° F. 127%  97%* 112%*  103%* *Denotes partial solubility of PipeRubber in Base Fluid.

Example 2

In this example, a well was experiencing significant loss of drillingfluid. Traditional loss prevention treatments with agents such ascalcium carbonate, fiber and graphite materials proved ineffective inreducing or stopping the loss.

A combination of 30 pound per barrel of crumb rubber with 20 pounds perbarrel of fiber were added to the drilling fluid. After the initialloading of crumb rubber and fiber, a maintenance load of 250 pounds ofcrumb rubber and 150 pounds of fiber per hour of pumping were used . Theresult was little or no additional loss of drilling fluid during therest of the drilling process.

Example 3

In this example, a deepwater drilling rig was experiencing fluid lossesof from 50 to 60 barrels an hour. Attempts to use conventional fluidloss control agents proved unsuccessful.

Two 50-barrel pills of fluid were prepared, each containing 15 poundsper barrel of crumb rubber were pumped into the well. After these pillswere pumped into the well, the rate of fluid loss dropped to between 10and 20 barrels an hour.

I claim:
 1. A drilling fluid additive comprising crumb rubber particles,said particles having a diameter of from 1 to 400 microns, and ahydrocarbon fluid.
 2. The drilling fluid additive of claim 1 whereinsaid crumb rubber particles vary in size from 20 to 400 microns.
 3. Thedrilling fluid additive of claim 1 wherein said crumb rubber particlehas a density of from 8.5 to 10.5 ppg.
 4. The drilling fluid additive ofclaim 1 wherein said hydrocarbon fluid is selected from the groupconsisting of crude oil, diesel oil, kerosene, mineral oil, gasoline,naphtha, toluene, ethylenedichloride, synthetic oils and mixturesthereof.
 5. The drilling fluid additive of claim 1 further comprisingcellulose fiber.
 6. The drilling fluid additive of claim 1 wherein saidhydrocarbon fluid is diesel oil.
 7. The drilling fluid additive of claim1 wherein such crumb rubber is capable of expanding to at least 40% ofits original size upon exposure to hydrocarbon fluids.
 8. A drillingfluid additive for reducing lost circulation of drilling fluidscomprising: (a) crumb rubber particles capable of expanding uponexposure to heat, said particles having a diameter of from 1 to 400microns; and (b) a hydrocarbon fluid.
 9. The drilling fluid additive ofclaim 8 wherein said crumb rubber particles have a density of from 8.5to 10.5 ppg.
 10. The drilling fluid additive of claim 8 wherein saidcrumb rubber particles are capable of expanding at least 40% of theiroriginal size upon exposure to a hydrocarbon fluid.
 11. A method ofreducing lost circulation and well comprising: (a) preparing a pill ofcrumb rubber particles and a hydrocarbon fluid, said crumb rubberpaticles having a diameter of from 1 to 400 microns; (b) injecting saidpill into said well; and (c ) forcing said pill into a lost circulationzone.
 12. The method of claim 11 wherein the crumb rubber has particlesvarying in size from 20 to 400 microns.
 13. The method of claim 11wherein said hydrocarbon fluid is selected from the group consisting ofcrude oil, diesel oil, kerosene, mineral oil, gasoline, naphtha,toluene, ethylenedichloride and mixtures thereof.
 14. The method ofclaim 11 wherein said hydrocarbon fluid comprises diesel oil.